The invention concerns ceramic graphic reflector blocks for use in nuclear reactors.
Reflectors are used in nuclear reactors to reduce losses by the migration of neutrons. As indicated by the name, at least part of the neutrons moving to the outside are to be reflected back into the fission zone of the reactor (the so-called reactor core). Due to the increased reactor flux at the edge of the fission zone as the result of the reflector effect, the power produced by unit mass of the fissionable material is increased, thereby leading to an improved utilization and thus more economical consumption of the nuclear fuel. The term "side reflector" designates the laterally placed reflector as distinguished from the bottom or roof reflector.
In gas-cooled high temperature reactors high purity graphite is used as the material of the reflectors. It is relatively inexpensive, has adequate mechanical strength and may be mechanically worked. In addition, it has good fire resistance and thermal conductivity. A disadvantage is the change in its crystalline structure caused by neutron and gamma irradiation and expressed by altered strength properties and volume.
Under the effect of temperature and high neutron fluxes, graphite initially undergoes a negative expansion, which changes into positive expansion with increasing fluxes, beginning at a point of reversal. This process is displaced with rising temperature toward lower flux values.
The differences in expansion within a structural part--as a function of the flux distribution, at the onset of the radiation the layers of the block close to the surface of the side facing the core tend to shorten more extensively than the deeper layers--are the cause of the generation of residual stresses. To reduce these residual stresses, measures to relieve them and to compensate for expansions must be provided. This is attained advantageously by means of slit surface structures, which signifies a dimensional reduction in parts of the block.
Recent developments in gas-cooled high temperature nuclear reactors, in particular those of low capacity (approximately 100 MWel) and correspondingly small core diameters, used for the shutdown of the reactor in place of adsorber rods inserted directly into the pile of spherical fuel elements, small absorber elements of a spherical shape, which are introduced into apprppriate cavities of the reflector. As is already known from the AVR in Julich, so-called nose stones comprising continuous vertical cavities are placed in the core, intended to contain shutdown elements. The nose stones are brick-shaped graphite blocks, extending radially from the side reflector with which they are physically connected, and projecting over the entire height of the reactor core.
Because of the aforementioned volume changes and the residual stress states generated by them in the irradiated graphite blocks, their surfaces on the side of the core are provided with vertical and horizontal surface slits, representing the resolution of the large original surface into small individual sections. To control the stresses in the nose stones, the cavities provided to receive the absorber elements are connected by gap-like, continuous openings with the core. The stresses in the nose stones are reduced by these openings to acceptable values. However, the aforementioned expansions lead in the course of the operation to a widening of the openings to such an extent that the separation of the absorber material and the fuel elements is no longer assured and absorber elements may leave the cavities and fuel elements may enter between them. By the appropriate setting of the geometric parameters of the connecting gaps, the absorber elements may be safely separated from the fuel elements.
It is possible, however, that cooling gas will enter the cavities from the core through the gap establishing the connection between the cavities and the core, and through other gaps that form in the course of operations between the individual nose stones as the result of radiation induced material deformations, with said gas then interfering with or even preventing the charging of the cavities with the absorber elements by a strong flow of gas. Such interferences are the consequence of the fact that the dead weight of the absorber elements is being overcome by the flow of gas and the absorber elements are placed into a suspended state. Based on this state of the art, it is the object of the invention to provide measures for the structural design of ceramic installations, which may be carried out both simply and cost effectively and which avoid the aforementioned shortcomings even during extended operations. In particular, the planned charging with absorber elements is to be assured without interference by the entering cooling gas. The known measures to reduce residual stresses generated by neutron induced deformations are to remain in force.